The present invention relates generally to suspension systems for motor vehicles which ensure improved braking performance of the vehicle during quick braking by control of the damping-force characteristic of shock absorbers.
Various suspension systems for motor vehicles are known which ensure improved braking performances of the vehicle during quick braking by control of the damping-force characteristic of shock absorbers. One of which is known in JP-A 3-109114 wherein control means are provided to vary the damping-force characteristic of a variable damping-force damper on the soft side during operation of a braking device. Specifically, control of the damping-force characteristic of the damper on the soft side during braking of the vehicle improves the pursuability of wheels with respect to the road surface, increasing the contact patch of the wheel. Thus, the road-surface frictional resistance which acts on the wheel is increased to effectively restrain a skid of the wheel, preventing lock of the wheel during braking of the vehicle.
However, the known system is constructed to control the damping-force characteristic of the damper always on the soft side during operation of the braking device regardless of other cruising conditions, producing the following problem:
With regard to an intermediate-frequency road wherein the road input frequency is principally in the band of an intermediate frequency between the resonance frequency of a sprung mass, i.e. mass above the spring and the resonance frequency of a unsprung mass, i.e. mass below the spring, control of the damping-force characteristic on the soft side is effective to improve the braking performance. However, with regard to a low-frequency road wherein the road input frequency is principally in the band of a low frequency corresponding to the sprung resonance frequency, and a high-frequency band wherein the road input frequency is principally in the band of a high frequency corresponding to the unsprung resonance frequency, such control enlarges variations of a sprung or unsprung mass, which increases wheel-load variations, resulting in deteriorated braking performance.
The reason will be described in detail. With regard to damper control during braking of the vehicle, a wheel load is one of the parameters of the suspension system which act on the braking system. Referring to FIG. 24, an influence of wheel-load variations upon the braking performance during antiskid control is such that with any road-surface frictional coefficient or road .mu., the greater is the wheel load, the longer is the braking distance. That is, a unique quantity of state which links the braking performance to the suspension performance is a wheel load, and its variations are variations of a sprung mass and a unsprung mass. And the braking distance tends to be prolonged with an increase in wheel-load variations.
In view of the fact that variations of the sprung mass and the unsprung mass are produced by a road input and an inertia force during braking of the vehicle, an influence of such conditions upon the braking performance will be described. When the sprung and unsprung masses vary with respect to road input, the braking performance is influenced by the damping-force characteristic of the damper. FIG. 25 shows a difference of the braking distance due to a difference of the damping-force characteristic and the input frequency. As seen in FIG. 25, when the input frequency is low, the soft damping-force characteristic damper has a longer braking distance. When the input frequency is between about 1.2 and 4.0 Hz, the hard damping-force characteristic damper has a longer braking distance. When the input frequency is 10.0 Hz or more the soft damping-force characteristic damper has a longer braking distance. It is understood that this reflects the transfer characteristic of the suspension system between the road surface and the sprung or unsprung mass. That is, in order to obtain optimum wheel-load variations with respect to road input by damper control, it is necessary to determine the road-surface conditions, i.e. the road input frequency.
Skyhook control will be described which is one of the effective means for insulating vibrations input from the road surface. As seen from its theoretical formula, skyhook control is a control to minimize variations of kinetic and potential energies of the sprung mass, and therefore it is effective for an improvement in the braking performance when a wheel load varies with variations of the sprung mass. However, when variations of the unsprung mass are active mainly as shown in FIG. 25, skyhook control is not concerned in a restraint of variations of the unsprung mass. Thus, control should also be carried out to restrain variations of the unsprung mass. Damper control for restraining variations of the unsprung mass can be obtained by increasing a damping coefficient. However, when applied to the road surface wherein variations of the unsprung mass are not active mainly, this control deteriorates the braking performance at the input frequency between about 1.2 and 4.0 Hz as described above.
It is thus understood that it is necessary to determine the conditions of the road surface on which the vehicle cruises, i.e. the road input frequency, as described above.
With regard to wheel-load variations produced by an inertia force which is active during braking of the vehicle, the magnitude of the inertia force corresponds to that of a braking force which is determined by how intense a driver depresses a brake pedal and how skiddy the road surface is. Thus, needed are determination of the brake operating state which indicates the operation amount of the brake pedal depressed by a driver and determination of the road .mu. which indicates the skidding degree of the road surface.
It is, therefore, an object of the present invention to provide suspension systems for motor vehicles which ensure optimum control of the braking performance in all conditions.